2. Dioda
As implies by its name diode is the vacuum
tube with 2 electrode, cathode and plate.
Sir J.A. Fleming (1849-1945) a British scienties
developed diode on 1904.
Simplified structure and schema of diode
seen on figure 7 above.
Cathode put on the centre of diode
surrounded by plate and heater inserted inside cathode.
When cathode heated by heater, electrons
will move from cathode to plate and produce plate current.
To get a clear idea about how diode works
let us observe 3 situations as follows :
- Dioda supplied by zero voltage
- Dioda supplied by negative voltage
- Dioda supplied by positive voltage
Diode Supplied by zero
voltage
When there is no voltage difference between
plate and cathode, heated electrons from cathode could never have enough
energy to reach plate, those electrons will start to accumulate near by
cathode forming electrons cloud. This is known as space charge. At this
situation there will be no plate current.
Dioda supplied by
negative voltage
If plate is made negative with respect to
cathode, the negative charge from plate will push back the electron
to cathode so the heated electron can not reach plate and still no current
indication appear at the amperemeter.
Dioda supplied by positive
voltage
If plate is made positive with respect to
plate, the positive charge from plate will attract heated electron from
catahode to reach plate and generate plate current. As the plate voltage
increase up the plate current will also increase.
Based on the diode property we have discused
above, which it can only flow plate only at certain voltage supply, so
diode is applicable for current rectifier application.
Dioda Plate Characteristic
The most important diode characteristic
is the plate characteristic, it shows the coorelation between plate voltage
and plate current.
Test circuit to achieve plate characteristic
and the example of plate characteristic seen on Figure 11 above.
Heater voltage supplied to filament to heat
diode until it reach certain temperataure (T1) and then plate voltage (Eb)
increased up from 0 until it reach maximum permisible diode plate voltage.
Plate current (Ib) will increase up as the
plate voltage increased, but when plate voltage reach a certain value plate
current standstill although the plate voltage still increased up. This
point known as saturation point.
But if diode temperature increased
up until it reach other higher temperature (T2), the increment of diode
voltage will follow back the increment of plate current. This situation
tells us that the change of diode temperature can influence it capability
to flow current.
Dioda Resistance
We have seen that plate current flowing
through a vacuum diode varies as the plate voltage is changed. Threrefore
diode may be considered as having internal resistance that limit the amount
of plate current flow. This internal resistance offered by diode is known
as its plate resistance. The resistance is not the same for direct current
as for the alternating current. Accordingly like any vacuum tube, diode
has two types resistance namely dc plate resistance and ac plate resistance.
DC plate resistance is the resistance offered
by diode to direct current. To get the clear idea about dc resistance let
see Figure 12A bellow
Figure 12.A shows the graph to measure DC
plate resistance. At point P of the curve the plate voltage
is OA and the corresponding plate current is OB, then the DC plate resistance
Rb is given by
Rb = OA / OB
As the plate characteristic is not a straight
line, therefore DC plate resistance is not constant but depend upon operating
point. So regarding to the diode real application DC plate resistance
must be determined at the actual operaating point.
AC plate resitance offered by diode to alternating
current and defined as The ratio of a small change in plate voltage
across a diode to the resulting change in plate current.
Figure 12.B shows us the way to measure
AC plate resistance. According to the definition of AC plate resistance,
the ac plate resistance rb is given by rb = BC/YZ
As tubes are generally used with AC voltage
than DC voltage so the ac plate resistance is more important than DC plate
resistance.